The file /etc/serialnum should contain the serial number
of your machine.

If /etc/serialnum does not exist, it is an indication that
your machine could not obtain the serial number from the
hardware. In this case you need to enter the serial number
manually. The serial number is found on the back of
the machine in the lower right hand corner. You should see
a tag that says:

NetworkAppliance SN: xxxx

Use a text editor to create /etc/serialnum and put the
machine's serial number in it. The file should contain a
single line that only has the serial number. The file is
used to help Network Appliance's customer service group
process your autosupport email more efficiently.

A warning is issued to the console if /etc/serialnum contains
a different value other than the hardware serial
number in which case it is automatically overwritten with
the hardware serial number. Also if the hardware serial
number and /etc/serialnum do not exist, then a warning is
issued to the console.